Demodulator



July 13, 1937.

B. s. McCl-JTCH EN DEMODULATOR Filed Marbh 16, 1936 2 Sheets-Sheet 1 I II I I OUD SPEAKER HUD/O AMPLIFIER RADIO nae-o UENC Y '5 AMP]. IFIERFIGURE I FIGUREIIA- PAD/0 FREQUE/VC Y INPUT 1?. M. S. VOLTAGE WWQQURUUVAR No) FIGURE 111 RAD/O FE5'QUCNCY INPUT FIGUREJI B- INVENTOR BRUNSONs. MfiUTCI- EN BYQQ AT TOR NEIY Patented Juiyilll", 1937 i PATENT pr ce.

2,087,063 nsMonULA'roa Bronson. McCutchen, Princeton Township, MercerCounty, N. J assignor to Alan N. Mann,

Scarsdale, N. Y.

Application March 1c, 1936, semi No. 69,071,

12 Claims.

My invention relates to demodulators for radio frequency currents; Morespecifically my invention relates to demodulators for radio frequencycurrents in which the demodulation is substantially linear for carrierinput currents below a predetermined value and in which no demodulationoccurs when the input'carrier currents ex- 1 .ceed the, predeterminedvalue.

I am aware of numerous demodulator circuits employing thermionic tubes,crystal rectifiers. and the like and especially those in whichbalancing, or limiting effects have been proposed. In general thesedevices have either distorted the desired signals, or failed to hold theoutput currents within the required limits.

One of the objects of my invention is in I means for demodulatingcarrier currents without distortion, and to prevent demodulation forcurrents which exceed a predetermined threshold value. Another object isto provide means in a demodulator which may be adjusted to establish athreshold value above which substantially no demodulation will result.Another object is to devise a radio frequency demodulator which willdetect modulated carrier currents but which will not detect impulses inexcess of predetermined values. An additional object is to automaticallyregulate the threshold value in proportion to the applied carriercurrents. A still further object is to provide a distortionlessdemodulator for desired signals and a balanced, or antidemodulator, forimpulses in excess of said desired signals.

Further objects of my invention and the theory and application of thecircuits may be best understood by reference to the accompanyingspecifications, and drawings in which,

Figure I is a circuit diagram of the demodulator of my invention appliedto a tuned radio frequency receiving set,

Figures HA and EB are graphs which illustrate the operation of myinvention, I

Figure III is a circuit diagram representing a modification of thedemodulator of Figure I,

Figure IV is a representationof a further modification of my invention,and a Figure V is a circuit diagram of the demodulator applied to asuperheterodyne receiver and illustrating one means for automaticallyestablishing the threshold value.

In Figure I, a suitable antenna system I is coupled to a tunable radiofrequency amplifier 3. The output circuit 5 of amplifier '3 is coupledto a tunable circuit 1 which is composed of an inductor 9 and a variablecapacitor II. The upper terminal of the tunable circuit I is connectedto the anode I3 of a diode rectifier I5. The cathode I! of thisrectifier is grounded. The lower terminal of the tuned circuit I isconnected to ground through a resistor I9, which is shunted 5 by acapacitor 2 I. I prefer to make the resistance of this resistor aboutone megohm, although I do not limit my invention to any particularvalue.

A second rectifier 23 is connected as follows: the cathode 25 isconnected to the anode I3 of rectifier IS; the anode 21 is connected tothe slider 29 of potentiometer 3i. A. battery 33 is connected across thepotentiometer 3i. The positive terminal of the battery is grounded asshown. An audio-frequency amplifier 35 is connected as follows: theinput is connected between the slider 3'I on resistor I3 and ground, andthe output is connected "to the loudspeaker 39 or other signalindicator.

, Although I do not intend to limit my invention to any particulartheory of operation, I believe the following description will be helpfulin understanding the operation of the circuits and the demodulator:Modulated radio frequency currents induced in the antenna I areamplified by the radio frequency amplifier 3. The amplified currentscreate voltages across the tuned circuit I. These alternating currentsmake the anode I3 alternately positive and negative with respect to thecathode I'I. When the applied voltage is below a predetermined value, apositive charge on the anode I3 causes electrons to flow from thecathode I! to the anode I3. Due to the wellknown unilateralconductivityof a diode rectifier, no electrons fiow from the cathode I!to the anode I3, when that electrode is negatively charged with respectto its cathode.

While no electrons will flow from the cathode I! to the anode I3 whenthe latter is negative, the 40 polarity is apparentlycorrect forelectrons to flow from the cathode 25 to the anode 21, because the anode21 is then positive with respect to' its cathode 25. However, noelectrons can flow in the diode 23 under the assumed condition, be-

2 cause the negative bias applied to the anode 21 from the battery 33 ismade to exceed the positive potential applied to the anode 21 by theincoming currents below the predetermined value. Thus under the assumedconditions electrons only flow from the cathode II to the anode I3 whenthe latter is positive as explained. These elections which doflow canonly escape to ground through the load resistor I9. The radio frequencycurrents are lay-passed to ground through the capacitor. M. The currentsthrough the load resistor establish biasing voltages which correspond tothe audio modulation of the incoming modulated radio frequency carrier.These audio voltages are applied in two ways: one as bias voltages tothe diodes I5, 23, (the grounded terminal of the resistor l9 being of apositive polarity); the o her as audio signalling voltages impressed onthe amplifier 35 through ground and slider 31.

The foregoing theory is in accord with the conventional explanation ofthe diode detectors with resistive load circuits, and explains theoperation of my device when normal desired modulated carriers areapplied to the receiving system. When excessive lmpulse's are applied,such as may be caused by static, the operation of the demodulator of myinvention may be best explained as follows: The excessive impulsesestablish voltages across the load resistor l9 which momentarily biasthe anode l3 of diode l5 and the cathode 25 of diode 23 negatively withrespect to the cathode l1 and anode 21. The incoming carrier voltage issuperimposed on this momentary bias. When the momentary bias attains avoltage which is somewhat less than half the steady bias of the battery33 as applied to the anode 21, a condition is reached in which some ofthe electrons passing to the anode i3 during positive cycles will passduring the negative cycles from the cathode 25 to the anode 21 and henceto ground through the potentiometer 3!. This direct path to groundprevents the building up of high voltages across the load resistor l9.When very strong impulses are received, the balancing action iscomplete, and the only current flowing in the load resistor will be puredirect current which produces no audio frequency output. Under suchconditions no demodulation takes place, and no audio frequency issupplied to the amplifier 35 to be reproduced in the loud speaker 39.

In Figure IIA a graph is shown which is based on measurements of appliedradio frequency input R. M. S. voltage plotted against voltage acrossthe bias resistor. The values are not absolute but relative for purposesof illustration. For increasing input voltages, the voltage across thebias or load resistor at first increases, gradually turns to a steadyvalue, and continues without indicating any change in voltage across theload resistor for greatly increased input voltages.

The foregoing is in accord with the explanation of the currents flowingin the bias or load resistor. Now examine the graph of Figure IIB. Aswas the case in Figure IIA, the present curves are based on measuredvalues. The demodulator of my invention was applied to a commercialradio receiver. The input voltages to the receiver were observed versusthe output voltages measured across the loudspeaker voice coil. Thevalues are shown as arbitrary units for purposes of illustration. Thevoltage output is substantially directly proportional to the appliedradio frequency input voltage from 0 to approximately 2. Just beyond thepoint 2 the curve begins to bend, then becomes flat, and beginning atabout. 6 starts to fall abruptly to a zero value of output voltage inaccordance with the explanation previously offered.

Viewing the two graphs together, it will be understood that the audiooutput shown on the curve of Figure IIB becomes zero and stays at thisvalue when the steady bias state has been reached as shown in Figure IIAfrom point 8 to IS. The zero output signal condition is recogniz'ed as abalanced condition rather than a limiter action. The balance conditionoffers an anti-demodulator, or silencer device for excessive static ornoise impulses exceeding the predetermined level.

In the matter of a proper steady bias for diode 23, it should beapparent that an optimum bias condition exists for every value of inputcarrier. If the bias is too low, strong desired signals will producedistortion, or the desired signals may be eliminated because they arenot demodulated. If the bias is too high, excessive impulses will not bebalanced out and will cause crashes in the' loud speaker. It is highlydesirable to automatically establish the optimum bias as will bedescribed. Manual control may be used for establishing the steady bias,or having established the bias, the signal amplitude may be adjusted tothe bias.

A modification of the demodulator of Figure I has been shown in FigureIII. The theory of operation of the circuit of Figure III does notmaterially differ from the preceding description. In this circuit, theprimary 4| of a tunable radio frequency transformer 43, is coupled tothe secondary inductor 45, which is shunted by a capacitor 41. Themid-point of the secondary inductor 45 is connected toa resistor 49which is grounded. The resistor is shunted by a' capacitor 5|. A slider53 connects the resistorto the input of an audio amplifier 55 whoseoutput circuit includes the loudspeaker 51.

The demodulator is connected as follows: The

normal detector is a diode whose anode 59 joins the upper terminal ofthe tuned circuit 45, 41

and whose cathode 6| is connected to ground.

The cathode 63 of a second diode is joined to the lower terminal of 45,41, and the anode is connected to the negative terminal of a biasingbattery 61. The positive terminal is grounded. The first diode rectifieris effectively connected across the upper half of the tuned circuit,while the second diode is connected across the lower half of thetunedcircuit. The load resistor 49 is serially connected with respect tothe biasing battery 61.

The first diode normally acts as the demodulator. The second diode hasan operating threshold which is determined by the effect previouslydescribed. The diode rectifiers in this and the preceding figures may beenclosed within a single envelope, or separate envelopes. In general aload resistorof about one megohm is preferred to lower values.

A modification of my invention is illustrated in Figure IV. In place ofa diode rectifier a grid detection triode is used. The input circuit isrepresented by a tunable radio frequency transformer H. The secondary ofthis transformer is comprised of an inductor I3 and variable capacitor15. The upper terminal of the tuned circuit 13, 15 is serially connectedthrough the grid leak resistor TI, grid capacitor 19. to the gridelectrode 8| of the thermionic triode 83. The cathode 95 is connected toground. The anode 81 is connected through the primary 89 of the audiofrequency transformer 9| to the positive terminal of the B battery 93.The negative terminal is grounded. The output of the transformer 9| isconnected to an audio fre-- quency amplifier 95 whose output may beconnected to any suitable indicator.

The means which prevents demodulation of v cuit is essentially the sameas Fig. I. In the present arrangement the demodulation of. normal signalrepresenting carriers is efiected by the rectification in thegrid-cathode circuit of .the triode. The operating threshold of thesecond diode 91 is determined by the voltage of the fixed bias minus therectified voltage across the grid leak resistor 11. The fixed biasvoltage may be obtained by a potentiometer or the like,

and in this as may be the case in the other circuits, the severalvoltages may be obtained from rectified, and filtered alternatingcurrent.

In the foregoing arrangements of demodula--' tor circuits a manualadjustment is made to determine the operating threshold, or value ofapplied voltages for which no demodulation takes place. I shall nowdescribe Figure V, which illustrates a circuit with automatic volumeconamplifier 201.

trol (hereinafter called AVC). Incomingca'rrier currents are amplifiedby the radio frequency The amplified output currents are detected in thefirst detector 203, and mixed with currents from the local oscillator205. The combination of these currents produces an'intermediatefrequency current which may be amplified by one or more stages 201 ofintermediate frequency amplification.

The' output circuit of the intermediate frequency amplifier 201 iscoupled to a resonant circuit comprising an inductor 209 and a capacitor211. 'The duo-diode section of thermionic tube 213 is connected acrossthe resonant circuit as follows: The anode 215 is connected to the upperterminal of the resonant circuit. The

tor 225.

lower terminal of this circuit is connected through the load resistor211 to the cathode 219. A capacitor 221 is shunted across the resistor211. The remaining anode 223 of the duo-diode is coupled to the firstanode 215 through a capaci- The anode 223 is also connected to groundthrough the resistor 221. The voltage for the AVG is developed acrossthis last mentioned resistor. Variations in the AVG voltage are filteredby a. network comprising resistor 229 and capacitor 231. The AVC voltagemay be applied through additional filters 233 to the radio frequencyamplifier 201, and intermediate frequency amplifier 201.

The audio frequency amplifiers are connected as follows: The gridelectrode 2351s grounded through a resistor 231 and is connected througha radio frequency choke 239 and capacitor 241 to the slider on loadresistor 211. The cathode 219 is biased positively with respect to thegrid 235 by lead 243 which is connected to a point of positive potentialon the potentiometer 245. The anode electrode 241 of the tube 213 isconnected through the primary of an audio frequency transformer 249. Thesecondary of this transformer is connected to the input of a secondaudio amplifier 251 whose output circuit includes signal responsivemeans. The B power source may be a battery 253 or any conventional powersource. Tl": several cathodes may be enpartially determines theoperating threshold as previously described in connection with the otherdemodulator circuits. It should be understood that the cathode 259 andanode 261 form the rectifier which cooperates with the normal detectorto prevent demodulation for voltages exceeding the threshold value.Instead of manually adjusting the threshold voltage, the grid electrode265 is connected to a slider 261 on the resistor 221 across which theAVG voltage is developed. The grid 265 is thus automatically biased bythe AVG voltage, and thereby automatically establishes the thresholdvoltage at which demodulation is prevented. The bias voltage applied tothe grid is negative and is equivalent to applying amuch larger voltageto the anode. In this connection an accurate, sensitive automaticthreshold control is obtained. Of course, the preliminary adjustment 'ismade manually.

.A demodulator and a method of demodulation have been described in whicha normal demodulation of modulated radio frequency carriers may beobtained while impulses exceeding a predetermined threshold value arenot demodulated. This system is very effective to prevent loud staticcrashes or the like in the loud speaker of a radio receiver. In thepreferred embodiment of my invention the relation between incomingcarrier currents and the threshold value is maintained by automatic biasmeans. My invention is not limited to the precise circuits shown,as-these circuits are examples and illustrations rather than limits,Numerous modifications will occur to those skilled in the'art;forexample, the demodulator applied to the superheterodyne may beapplied to a tuned radio frequency'recelver, and vice versa. Likewisethe triode grid detector may be used with any type of recelvenand othertypes of thermionic tubes or rectifiers may be 'used. The bias voltagesmay be applied to bias the cathode positively, with respect to anodewhich is then grounded, instead of biasing the anode negatively withrespect to the grounded cathode as shown.

I claim:

1. The method of demodulating normal modulated carrier currents andpreventing demodulation of excessive impulses by means of a normalsignal rectifier, an opposing rectifier for excessive impulses normallybiased to prevent its cessive impulses in said opposing rectifier byautomatically reducing the bias of said opposing rectifier, applyingrectified currents from both rectifiers across said common load circuitwhereby no substantial demodulation occurs for said exces sive impulses.

2. The method of demodulating modulated carrier currents and preventingdemodulation of excessive impulses by means of a normal signalrectifier, an opposing rectifier for excessive impulses normally biasedto prevent its operation, and a common output circuit for saidrectifiers which comprises rectifying normal modulated carrier currentsand excessive impulses in said normal signal rectifier, rectifying saidexcessive which comprises rectifying normal. modulatedcarrier currents,rectifying saidex impulses in said opposing rectifierby automaticallyreducing the bias of said opposing rectifier, applying rectifiedcurrents from both rectifiers to said common output circuit whereby nosubstantial demodulation occurs for said excessive impulses.

3. The method of demodulating normal carrier currentsand preventing thedemodulation of impulses exceeding a predetermined limit which comprisesrectifying normal carrier currents, rectifying both phases of excessiveimpulses applying the rectified phases of such excessive impulsesthrough a common load circuit so'that such phases oppose each other, andpreventing the rectification of both phases of normal carrier currents.

4. Themethod of demodulating normal carrier currents and preventing thedemodulation of impulses exceeding a predetermined threshold whichcomprises rectifying normal carrier currents, rectifying both phases ofexcessive impulses, preventing the rectification of both phases ofnormal carrier currents, and automatically establishing the saidthreshold in accordance with incoming carrier currents.

5. The method of demodulating carrier currents and preventing thedemodulation of excessive impulses in accordance with claim 2 plus theadditional step of automatically adjusting the bias of said opposingrectifier in accordance with incoming carrier currents.

6. A demodulator comprising an input circuit, a normal signal rectifierconnected thereto, a second circuit connected with such rectifieradapted to be traversed by the rectified current from such rectifier, asecond rectifier connected with said circuits and so disposed as to feedvoltage of reversed phase to said second circuit as compared with thevoltage from said first rectifier biasing means tending to render saidsecond rectifier inoperative under normal conditions, and means forcausing potential to build up in said second circuit tending tocounteract said biasing means.

7. A demodulator comprising an input circuit, a normal signal rectifierconnected thereto, a

load circuit connected with such rectifier, a

I second rectifier connected with said circuits and a normal signalrectifier connected thereto, a second circuit connected with suchrectifier comprising a resistance shunted by a capacity, asecond-rectifier connected to receive currents from said input circuitand adapted to transmit rectifled currents. to said second circuit,biasing means for limiting the operation of said second rectifier tovoltages of predetermined values, and means for automatically buildingup voltage in said second circuit tending to counteract said bias.

9. A device for demodulating carrier currents and preventingdemodulation of excessive impulses'comprising an input circuit, twodiode rectifiers connected across said input circuit in mutuallyreversed relation, a load circuit comprising a resistance and capacity,means for connecting said diodes to said load circuit to delivervoltages to said load circuit in opposed phase relation and biasingmeans for establishing a threshold for one of said rectifiers.

10. A device as specified in claim 9, in which the resistance andcapacity comprised in the load circuit are of such values as to producea time factor in such load circuit of sufiicient magnitude so thatvoltages produced by successive pposite phases will oppose each other.

lL'A device for demodulating carrier currents and preventingdemodulation of excessive impulses, comprising a normal signal rectifierfor rectifying normal carrier currents and excessive impulses, anopposing rectifier for rectifying excessive impulses normally biased toprevent its operation, means for automatically reducing the bias of saidopposing rectifier so that such opposing rectifier will serve to rectifyexcessive impulses without rectifying normal carrier currents, a commonload circuit for said rectifiers, and means adapted to apply rectifiedcurrents from each of said rectifiers to said common load circuitwhereby no substantial demodulation occurs for excessive impulses.

12. A device for demodulating normal carrier currents and preventing thedemodulation of impulses exceeding a predetermined threshold, whichcomprises means for rectifying one phase of normal carrier currents andof excessive impulses, a second rectifying means for rectifying theopposite phase of excessive impulses, means to establish a threshold toprevent the rectification ofv the opposite phase of normal carriercurrents by said second rectifying means, means whereby such thresholdis modified in accordance withincoming carrier currents,.and a commonload circuit for both of said rectifying means.

BRUNSON MCCUTCHEN.

